This invention relates to semiconductor memory devices and more particularly to an improved high-density MOS random access, dynamic read/write memory.
One of the most widely used memory devices for computers is the MOS dynamic RAM of the 64K-bit size illustrated in U.S. Pat. No. 4,239,993, issued to McAlexander, White and Rao, assigned to Texas Instruments. As memory density increases to the 256K-bit and 1-Megabit sizes, the number of storage cells connected to a single bit line half would also increase. With an increased number of cells, the bit line capacitance would also increase. Because of upper limits on the chip size, the storage cell capacitance has tended to remain relatively constant, or possibly decrease. Since the signal available on the bit lines for sensing is a direct function of the ratio of the storage cell capacitance to the bit line capacitance, this signal would also tend to decrease with increasing cell density. One solution to this problem is to use a more sensitive sense amplifier, but increased sensitivity demands increased sensing time, and also the smaller signal increases probability of alpha-particle induced soft errors. Another solution is to reduce the number of cells on a bit line and to share a sense amplifier between two pairs of bit line halves, requiring isolation of the halves not being read from. Along with this multiplexed or shared sense amplifier scheme comes problems with data propagation time through the sense amplifier, particularly during a "write" cycle.
It is the principal object of this invention to provide an improved high-speed, high density, dynamic random access memory, particularly for multiplexed sense amplifier configuration. Another object is to provide a semiconductor dynamic memory device which uses multiplexed sense amplifiers with no speed penalty. A further object is to provide an improved shared sense amplifier type of dynamic RAM circuit which overcomes difficulties in write cycle timing, and in writing a full supply level into the selected storage cell.